1. Field of the Invention
This invention relates to a toner composition for use in electrophotography. More particularly, this invention relates to a toner composition for use in electrophotography which, in both indirect and direct developing methods of electrophotography, is uniformly charged with a strong positive electric charge thereby permitting clear development of negative electrostatic latent images without any fogging.
2. Description of the Prior Art
As is well known, xerography is a type of electrophotography which uses static electricity. It is disclosed in U.S. Pat. No. 2,297,691. Likewise there have been disclosed several methods (developing methods) of developing or making visible the electrostatic latent image which is formed on the surface of an insulating substance such as, for example, the cascade method disclosed in U.S. Pat. No. 2,618,552, and the magnetic brushing method disclosed in U.S. Pat. Nos. 2,874,063 and 2,764,441.
In the cascade method there is employed a colored resin powder (toner) and glass beads having a coarser particle size used as a carrier. A mixture of the toner and the carrier are dusted onto a sheet. In the magnetic brushing method, there are employed a toner and powders of magnetic substances such as iron powder used as a carrier. This carrier can be agitated by magnetic force. In both of these two methods the toner particles are electrically charged by friction and then they are attached onto an electrostatic latent image by means of electrostatic attraction.
As noted above, the toner is a colored resin powder. Generally, the following properties are required for a toner:
1. particles charged by friction by the cascade method or the magnetic brushing method should be charged uniformly either completely positive or completely negative; if both positive and negative charges are co-present in the same toner, there will occur such defects as the whiteness of the non-image background will be reduced, a trailing phenomenon will be observed, the co-presence of opposite images (co-presence of negative and positive image) will be seen, and indistinct edges will be formed on the developed images; PA1 2. toner particles charged by friction should possess a sufficiently strong charge; otherwise the toner will not be strongly attracted to the image area and it will be dispersed so as thereby to pollute the inside of the developing apparatus; PA1 3. electrification (or charging capability) of the toner should not be influenced greatly by humidity, rather it should always be substantially constant under varying humidity conditions to ensure stable charging; PA1 4. desirably the toner should melt sharply at a relatively low temperature, but it should not cause blocking or be sticky at normal (20.degree.-25.degree. C) temperature. If a viscous resin is employed, the resulting toner will stick to the surfaces of the carrier and the photosensitive layer and will not provide a sharp and clear developed image. Accordingly, the life of the toner is also shortened as is well known; PA1 5. the surface of the photosensitive layer should readily be washed; PA1 6. it should be easy to pulverize; PA1 7. it should have good abrasion resistance; PA1 8. it should have good fixability to a sheet of paper to form a copy; PA1 9. it should have proper fluidity; PA1 10. the resin and the colorant should be capable of being mixed well so that the colorant can be dispersed uniformly in the resin; and PA1 11. it should not generate an unpleasing smell or poisonous gases at the time of heat fixing. PA1 a. from 55 to 100 % by weight of a polyester resin (first resin component) which has a softening point of 80.degree. to 130.degree. C. measured in accordance with ASTM-E28-51T (ring and ball method), which has a nitrogen content of from 0.2 to 2.5 %, based on total weight of the polyester resin, and which is obtained by the reaction of; PA1 b. the balance of the resin composition, i.e. from 0 to 45 % by weight of said resin composition, is a resin (optional second resin component) which has a softening point of 80.degree. to 150.degree. C. and is selected from the group consisting of polystyrene, rosin maleate, petroleum resin and epoxy resin.
When the non-exposed portion of the surface of the photosensitive layer, prior to the developing step, is charged with a negative electric charge after electrification and exposure, a positive picture image cannot be obtained unless it is developed with a toner which is charged with a positive electric charge. Most resin powders imparted with friction by the cascade method using glass beads as the carrier or by the magnetic brushing method using iron powder as the carrier are charged with a negative electric charge, and very few are charged with a positive electric charge.
As positive charging toners, there have heretofore been proposed several compositions wherein a colorant (e.g. carbon black) and a charge-control agent are blended with a thermoplastic resin. A two-component system toner consisting of a colorant and a thermoplastic resin such as polyester or polystyrene resin and not containing a charge-control agent, can be charged with only a very small negative charge or positive charge by friction thereof with the carrier. In order to impart to the toner a strong negative charge or strong positive charge, there have also been proposed, and used in practice, several methods. To give a strong negative charge, for example, in the method disclosed in Japanese patent publication No. 26478/70 an azo-type oil soluble dye containing a complex of chromium is added as a charge-control agent. Alternatively, in order to give a strong positive charge, a basic dye is added.
Generally, however, these additives do not admix well in a thermoplastic resin; hence, uniform electric charging, either negative or positive, on the surface of the toner particles can scarcely be effected. This causes such major problems as fogging of the images at the time of development, trailing phenomenon, or indistinct images (blurred images), and so forth.